The present invention relates to electrodes for electrolytic capacitors and, more particularly, to a method for producing such electrodes, particularly anodes but also cathodes, having increased surface area, and the electrodes so produced.
An electrolytic capacitor is a capacitor in which one plate is metallic and the other plate is an electrolyte. Intervening between the two plates is a dielectric consisting of a surface oxide coating on the metal plate. Conventionally, the metal plate on which the dielectric coating is formed is referred to as the anode. The term "anode" is used herein to refer both to the metal plate itself and to the combination of the metal plate with the dielectric coating. It will be clear from the context which meaning of "anode" is intended. A transition between ionic conduction in the electrolyte and electronic conduction in an external circuit is provided by a second metal plate which is referred to herein as the cathode. The anode and the cathode are referred to herein collectively as electrodes. Typically, the anode and the cathode are thin foils.
Typically, the metal of the anode is a valve metal, i.e., a metal which, when oxidized, allows current to pass if used as a cathode but opposes the flow of current when used as an anode. Examples of valve metals include magnesium, thorium, cadmium, tungsten, tin, iron, silver, silicon, tantalum, titanium, aluminum, zirconium and niobium.
As is the case with capacitors generally, the capacitance of an electrolytic capacitor is proportional to the surface areas of its two plates. Conventionally, the surface areas of the foils are increased by etching. In the case of very thin foils, this has the disadvantage of weakening the foils mechanically, making it difficult to fabricate electrolytic capacitors by high speed winding.
Recently, vacuum deposition has been proposed for increasing the surface areas of foil electrodes. Kakinoki et al., in U.S. Pat. No. 4,970,626, report the vacuum deposition of titanium on aluminum foil, at an angle, to produce a titanium surface with a rough, columnar structure. The disadvantages of this method of increasing the surface are of a foil electrode include the additional cost of working with two metals and the decrease in stability associated with an intermetallic potential. Drake, in U.S. Pat. No. 4,309,810 also teaches the vacuum deposition of a metal vapor at a low angle onto a foil substrate, and presents an example of the deposition of aluminum on aluminum to give a columnar structure. Drake's foil has been found to be too brittle for use in electrolytic capacitors: it breaks when it is rolled into a cylindrical roll, the standard shape of an electrolytic capacitor. Neumann et al., in German Patent No. 4,127,743, report the vacuum deposition of aluminum on aluminum in a low pressure oxygen atmosphere, to give a surface structure of columns of aluminum separated by aluminum oxide. Allegret et al., in U.S. Pat. Nos. 5,431,971 and 5,482,743, also report the codeposition, under a low pressure oxidizing atmosphere, of a mixture of aluminum and aluminum oxide. Such mixed Al--Al.sub.2 O.sub.3 surfaces are more robust mechanically than pure aluminum surfaces; but electrolytic capacitors incorporating them are known to have relatively high resistive losses and relatively low stability over time. In addition, the presence of both aluminum and large quantities of aluminum oxide in the surface of the foil makes difficult and less effective both stabilization by subsequent conventional chemical or electrochemical treatments and structure coarsening by subsequent annealing.
Having formed a foil electrode with high surface area, if the electrode is to be used as an anode, its surface must be oxidized. Conventionally, this is done by electrolytic anodization, in which the electrode is used as an anode in an electrochemical cell. Recent patents in this art include U.S. Pat. No. 4,537,665 to Nguyen et al., U.S. Pat. No. 4, 582,574, to Nguyen et al., and U.S. Pat. No. 5,643,432 to Qiu. The thicker the oxide layer, the lower the capacitance of the electrolytic capacitor, but the higher the working voltage of the dielectric. For high voltage (upwards of 100V) applications, the dielectric layer is relatively thick, and tends to bridge over fine surface features, reducing the effective surface area of the anode.
Two other phenomena tend to reduce the effective surface areas of anodes made by electrolytic anodization. One is that in the course of the anodization process, oxygen and hydroxide ions migrate from the metal-dielectric interface into the metal, while metal ions migrate from the metal-dielectric interface into the dielectric. The other is that sharp points on the metal surface are characterized by high local electric fields, which accelerate the electrolytic process. Both of these phenomena tend to smooth out irregularities in the metal-dielectric interface.
There is thus a widely recognized need for, and it would be highly advantageous to have, improved methods for creating mechanically robust foil electrodes of high surface area.